Forms :: Retrieving Previous Record Value Of Same Item In A Block

I am creating a record with the items - S_No, Product_Name, Selling_Qty, Price.

Checking the total qty (Tot_Qty) from the purchase stock - table1 and already sold out qty (Sold_Qty) of the product from table2.

Consider the following values: Tot_Qty = 200; Sold_Qty = 50;

Now i create a first record of the form as follows: here on selecting the Product_name from LOV, will display the Selling_Qty item - which can be edited (Selling_Qty = Tot_Qty - Sold_Qty) and its price.

S_No Product_Name Selling_Qty Price
1. KeyBoard 50(150) 200
2. KeyBoard 30(150) 200
3. KeyBoard 50(150) 200

Note:
[value given within () are displayed automatically after selecting the prod]

My prob is, when i am entering next record, it displays the same 150 as qty, which have to be 100. After selecting the product from LOV, it should calculate & validate as shown below: which means, previous record value of Selling_Qty also have to be subtracted and displayed.

S_No Product_Name Selling_Qty Price
1. KeyBoard 50(150) 200
2. KeyBoard 30(100) 200
3. KeyBoard 50(70) 200

Is there any built-in functions to retrieve previous record value?